Is awareness of Joe Biden’s foreign corruption the main reason President Obama put off endorsing his candidacy until so late in his campaign?

Your intuition has served you nicely here. While no one can prove what is in someone’s heart and there are always many political discussions over months and years about expectations, plans, and possible scenarios of all kinds, in his heart of hearts Obama knew that Joe Biden was a kind of timebomb that could explode at the wrong time and undermine the continuation of a Democratic regime, and is the true reason for the obvious reluctance he showed in backing his former vice president.